A porous film is used for various applications, such as hygiene material, medical material and battery separator. In the case of using a porous film as a battery separator of a lithium ion secondary battery or the like, high puncture strength is required.
As a porous film superior in the puncture strength, it is known to use a porous film obtained by kneading a high-molecular-weight polyolefin having a weight average molecular weight of 5×105 or more, a thermoplastic resin having a weight average molecular weight of 2×104 or less, and a fine particle; shape-forming the kneaded product into a sheet; and then stretching the sheet (see, JP2002-69221A (Patent Document 1)). Such a porous film can be easily obtained from the polyolefin having a high molecular weight, without using an organic solvent in the production process, and the obtained porous film is superior in the puncture strength.
Also, it is known to obtain a laminated porous film as a battery separator by laminating a porous heat-resistant layer on a porous film made of a thermoplastic polymer (see, JP2000-30686A (Patent Document 2)). A battery using this laminated porous film as the separator is superior in safety, since even when the porous film melts upon extraordinary heat generation due to overcharge or the like, the heat-resistant layer can prevent contact between the positive electrode and the negative electrode.
Such a laminated porous film can be obtained by a method of applying a solution containing a heat-resistant resin on a porous film and then volatilizing the solvent from the solution to form a heat-resistant layer, or by a method of applying the above-described solution on a porous film and then exposing the resultant to a poor solvent to precipitate the heat-resistant resin. These methods are simple and superior in productivity. However, the laminated porous film obtained by such a method is required to have more improved adhesive strength between the heat-resistant layer and the porous film.